All ores, whether metallic or non-metallic, are aggregates of minerals in which the valuable portion to be recovered is generally mixed with foreign material called gangue. The separation of the valuable from the worthless portion is the art of ore dressing and employs such processes as, for example, jigging, tabling, cyanidation and flotation. A preliminary step to all the methods of ore dressing is the crushing and grinding of the ore to liberate the valuable material from the waste. The ore is ground, or comminuted, until the valuable material is broken free from any attached or enclosing gangue. The class of the ore to be treated determines the subsequent ore dressing treatment after comminution.
In the gravitational separation of values from ores, the difference in specific gravity between the various materials composing the ores is utilized. Gravitational separation of materials of different densities by procedures such as sink-float processes is well known and is based on the law of physics that the loss of weight of a body immersed in any liquid equals the weight of the liquid displaced. Accordingly, bodies lighter than the weight of the volume of a liquid they displace will float on it, while those heavier than the weight of the volume of a liquid they displace will sink in it. Therefore, if an ore is sufficiently ground to free the valuable material and is immersed in a parting liquid having a specific gravity intermediate the specific gravities of the valuable and worthless portions, a positive separation can be achieved and either the "sinks" or the "floats", or both, may be recovered for its values.
As-mined coal comprises pure coal having a specific gravity of about 1.3 and solid foreign matter, such as rock, slate and pyrite, having a specific gravity typically from 1.7 to 4.9. These components occur in all combinations, so that the as-mined coal is a very heterogeneous material. By introducing the crushed as-mined coal into a parting liquid having a specific gravity intermediate that of the coal and foreign material, the coal can be recovered as "floats" apart from the gangue.
The parting medium employed may be a mixture of some finely divided solid matter suspended in water in which case there must be a constant agitation of the liquid in order to maintain a suspension. This suspension of particulates such as sand, magnetite or barite provides a parting medium that is heavier than water. The specific gravity of this type of parting medium can be controlled by varying the proportions of the solid matter and water to suit optimum conditions in the coal cleaning.
The use of non-aqueous heavy-gravity liquids is also well known in the art of beneficiating coal by the sink-float process. In most part, these non-aqueous heavy liquids are halogenated hydrocarbons, particularly those containing from 1 to 2 carbon atoms and from 2 to 6 halogen atoms. Reference may be made to U.S. Pat. Nos. 2,150,917 and 3,348,675 for listings of some of the halogenated hydrocarbons that can be used as heavy parting liquids which are hereby incorporated by reference.
In "Sink-and-Float Separation Commands New Attention," Engineering and Mining Journal, May 1938, Vol. 139, No. 5, W. B. Foulke discusses the general properties that a heavy liquid should possess. Minimum miscibility with water is most important in that any heavy liquid in which water is soluble would decrease in specific gravity during use because water, which is generally associated with the mined and the ground ores, would be continually extracted by the heavy liquid. Accordingly, some means would have to be provided to return the heavy liquid to its proper specific gravity. Substantial immiscibility with water is a property common to the halogenated hydrocarbons, thus making them attractive parting liquids.
Because the desired specific gravity for the heavy parting medium necessary for a particular gravity separation may not be possessed by any single halogenated hydrocarbon, the practice is to mix two or more such heavy liquids in proportions that afford a parting medium of the chosen specific gravity. Alternatively, hydrocarbons, either aliphatic or carbocyclic, of low specific gravity can be added to a halogenated hydrocarbon to lessen the specific gravity of the parting medium.
There are problems with this method of adjusting the specific gravity of a halogenated hydrocarbon heavy liquid by adding an amount of another halogenated hydrocarbon or unsubstituted hydrocarbon. Since halogenated hydrocarbons are expensive relative to unsubstituted hydrocarbons, a second halogenated hydrocarbon for diluting purposes is undesirable. On the other hand, while halogenated hydrocarbons are generally non-flammable, dilution with highly flammable hydrocarbons creates a serious safety question in the construction of material handling and electrical equipment. Suitable liquids for adjusting the specific gravity of a heavy parting liquid while avoiding these problems are not known.
Thus, there is a need for a new heavy parting medium whose specific gravity can be easily adjusted without the need for addition of halogenated or unsubstituted hydrocarbons.
There is also a need for a new heavy liquid that does not present flammability safety problems.
There is yet a need for a new heavy liquid whose specific gravity can be readily adjusted over a range of about 1.30 to 1.50 to beneficiate various metallurgical coals at a specific gravity less than 1.50.
There is a further need for a heavy liquid for use in making coal separations which possesses low viscosity and surface tension, ease of density adjustment, ease of regeneration for re-use and is non-destructive to the coal, non-flammable, non-toxic and non-contaminating.
There is a still further need for a method of adjusting the specific gravity of a substantially water-immiscible parting liquid by the addition of water.
There is yet a further need for a new parting medium comprising a water-immiscible liquid and water the specific gravity of which can be readily varied to desired values.